Gas sensor

ABSTRACT

The invention comprises a gas sensor (1), designed to enable a measurement of a gas sample that is enclosed in a cavity (2), having the shape of a block (50); the wall or wall sections of the cavity exhibiting highly-reflective properties for light. Said cavity contains means (51) for incoming rays of light, and means (52) for outgoing rays of light. Said cavity (2) exhibits opposing surface sections (55a, 56a, 57a) that are designed and coordinated so that the incoming rays of light (60) are arranged to pass--without reflecting, or without appreciably reflecting within a plane (the x-z plane)--across said cavity (2) to a concave mirror surface (55a) that is oriented in a right angle to the plane; that said rays of light are arranged to pass reflected across said cavity (2) in a plane of the mirror surface to the other surfaces (56a, 57a), thereby forming a measuring path before the reflected beams of light are aimed towards said means (52) for exiting light.

TECHNICAL FIELD

The present invention relates to a gas cell and/or to a gas sensor.

By gas cell we mean a cavity in which a gas sample exists, which gassample is penetrated by a beam of light or by rays of light, to enablean analysis of the gas structure and concentration by means of thechange of the spectrum of the penetrating beam of light, by means of alight source and/or a light detector, which are positioned outside ofthe cavity, or the unit in which the cavity is formed.

If the unit is permitted to contain said light source, light detector,and/or circuits for evaluating gas structures and/or gas concentrations,then it is called a gas sensor.

For the sake of simplicity, the description that follows and the Claimssection use the expression gas sensor for both types of design.

In particular, the present invention relates to a gas sensor that isdesigned to enable a measurement of a gas sample enclosed in a cavity.

According to the invention, the design of the gas sensor comprises ablock, which contains a cavity that serves as a gas cell. As relates tolight, the wall or wall sections of the cavity exhibit highly-reflectiveproperties.

Said cavity contains means, such as an opening or section for incomingrays of light or light, and means, such as an opening or section forrays of light or light to be received within or exit the cavity afterthe light has been reflected one or more times inside the cavity.

In particular, the present invention provides a gas sensor, which ispart of a system wherein a gas and/or a portion of gas in a gas sampleis analysed with respect to the gas itself and/or to the proportionalshare (concentration) of the gas in the sample, using an absorptionspectrum, which appears in the light spectrum of the exiting light.

To better understand the present invention and its characteristics, weare required to use expressions such as directly-reflected light beam orrays of light, and indirectly-reflected light beam or rays of light.

By directly-reflected light beam or rays of light we mean that a beam oflight may pass through the cavity without reflecting in the cavity'sdelimiting parallel surface sections, except in a mirror surfaceopposite the beam of light, which reflects the beam of light in the sameplane, but in a different direction.

By indirectly-reflected light beam or rays of light we mean that a beamof light may pass through the cavity reflecting one or more times in themirror structure of the cavity's delimiting surface sections.

BACKGROUND ART

Gas sensors of the kind described above exist in various differentembodiments.

These kinds of gas sensors have a cavity or space that functions as agas cell through which rays of light are allowed to pass. Interactingwith the gas cell are light emitting means and light receiving means(light receiver). The light receiver is designed to enable an evaluationof the current lines of absorption in the light spectrum for an exitingbeam of light, or rays of light.

Within the cavity, between the light emitting means and the lightreceiver in the gas cell, is a light path, hereafter called a measuringpath or optical measuring path.

In optical applications the terms "geometrical path" and "optical path"are sometimes used, where the geometrical path is a geometrical distanceand the optical path is a geometrical distance multiplied with therefractive index of the medium through which the light passes. In socalled "standard air", where the refractive index is 1, the optical pathis thus equal to the geometrical path.

The terms "measuring path" and "optical measuring path" are usedsynonymously in this description since the gas concentrations at handare very low which gives a refractive index very close to 1.

Even if the refractive index would deviate substantially from 1, forexample at a measurement of a fluid, these terms could be usedsynonymously since the used technique is based on reflections of lightin a homogeneous medium and not on transmissions of light throughvarious mediums with different refractive indexes, wherefore thecompensations for variations in the refractive index is not required. Itis nevertheless obvious for a person skilled in the art what measuresare required if the light emitting means and/or the light receivingmeans are positioned in an optical medium that differs in refractiveindex from the medium inside the gas cell.

It is also known that thorough analysis of gas type, gas mixture, and/orgas concentration in a gas sensor with a gas cell is based on thefollowing relationship: when concentrations are low, and when theabsorption spectrum for a given gas is not expressly obvious, then along measuring path is required for a light beam that passes through agas sample in order to achieve an accurate result.

Today, a measuring path of approximately 0.1 meters is required in mostpractical applications that use current technology to produce beams oflight, to detect and receive beams of light after they have beenreflected a given number of times, and to analyse the absorptionspectrum for the beam of light.

The U.S. Pat. No. 5,163,332 shows and describes a gas sensor that can beused to analyse gases. The gas sensor consists of a long, hole-shapedpipe with internal light reflective surfaces that allow the pipe tofunction as a channel or conductor of light, thereby creating ameasuring path for transferring beams of light from a light source to adefector, and through a gas sample enclosed in the pipe.

The beams from the light source are arranged to be indirectly reflectedby opposing surfaces--that is, by indirect reflection in all directions.

By reflection in all directions in an optical conductor, we mean that ifrays of light, coordinated into a beam of light with diverging beams areallowed to enter a pipe whose inside has light-reflective properties,then the beams of light that angle away from the centre line, thez-axis, will be reflected in the x-z plane as well as in the y-z plane.

Further, several openings in the wall section of the pipe are shownequipped with filters, which permit a gas sample to freely be introducedinto the pipe, or to freely exit from inside it.

Attempts have been made to reduce the external dimensions of a gassensor or gas cell while obtaining a long measuring path.

The following publications are referred to as examples of the backgroundart.

The U.S. Pat. No. 5,340,986 showed a diffusion type gas sensor, wherethe required length for the gas cell can be reduced by half, relative toa desired measuring path, by arranging a transmitter and a receiver inone end of the pipe, and a mirror in the other end of the pipe, as wellas by giving the inside of the pipe light-reflective properties.

This method offers a directly-reflected light beam andindirectly-reflected rays of light.

The U.S. Pat. No. 5,060,508 made known a gas cell with an extendedmeasuring path--relative to its external dimensions--shaped in a blockwith small external dimensions. The block is equipped with several canalsections, oriented at the front and back, and connected to one another.The walls for these connected canal sections are coated with ahighly-reflective agent, causing the resultant passage to serve as anoptical conductor, in order to transfer beams of light by means ofindirect reflection in all directions. Several minor passages permit thegas in an area surrounding the pipe to diffuse into the passage.

In this example, the gas cell is created by positioning the two blockhalves against one another. These halves can be cast of plastic, makingthem relatively inexpensive to produce.

The content of the publication EP-A1-0 647 845 is also a part of thebackground art in this regard.

Considering the characteristics exhibited in the present invention, weshould also mention that an absorption cell was previously made known byJ. U. White, in the Journal of Optical Society of America, Vol. 32, page285 (1942). A cell was shown and described, consisting of threespherically concave mirrors, each of which had the same radius ofcurvature, and was positioned to create a desired optical measuringpath.

The cell was developed in order to obtain an extended measuring path fora directly-reflected beam of light in a gas sensor. By applying theprinciples described in the above-mentioned publication, it has beenpossible to develop gas sensors whose optical measuring paths exceed 10meters in length.

Through this example, it is known to position the mirrors far apart,ordinarily 0.3 to 3 meters, and adjust the rays of light so that theirdiverging angle is very slight.

Further, the rays of light produced must not be interfered with byindirect collateral reflections. Instead, they must be reflecteddirectly between the mirrors.

The U.S. Pat. No. 4,756,622 describes how other measures have been takento create a long measuring path for wave length absorption in gas. Forexample, this publication explains that the light beam may wanderthrough a limited volume of gas very many times; for example in theorder of thousands of times.

The light beam is introduced to a closed optical loop, where it isallowed to circulate through the gas sample. Then, after havingcirculated through the gas sample a predetermined number of times, thelight is deflected from the closed optical loop by means of polarisingwave conductors.

Even when the reflective factor is as high as 0.998, after 100reflections the intensity of the light is only 80% of its originalbrightness. After 300 reflections, it is only approximately 50% of itsoriginal brightness.

DISCLOSURE OF THE PRESENT INVENTION Technical Problems

Given the background art, as it has been described above, allowing thatsome gases may exhibit a weak tendency towards absorption, so that theycan be analysed only after rays of light are brought to pass through arelatively long optical measuring path through the gas; and/or allowingfor other situations in which the analysed gas may exhibit a distincttendency towards absorption, but must be detected in very lowconcentrations--typically in parts per million (ppm) or less--therebyrequiring a long measuring path, it ought to be considered a technicalproblem to be able to miniaturise the gas cell, by simple means, whileat the same time providing an adapted and chosen measuring path and/ormeasuring paths.

Another technical problem is in being able to create a gas cell and/or agas sensor that can be developed as an inexpensive mass-producible unit.

Another technical problem is in being able to provide a gas sensor thatis so easily adjusted that it can reliably analyse poisonous gases (suchas carbon monoxide, ozone, nitrogen oxide, and so on) with a detectionof low concentrations in the range of ppm.

Another technical problem is in being able to create an inexpensive gassensor that can function as a fire detector, being adapted toefficiently and accurately be able to analyse low concentrations ofcarbon dioxide and carbon monoxide.

Yet another technical problem is in being able to realise the measuresthat must be taken to create a gas cell or a gas detector through whicheven diverging beams of light in a first plane (for example the z-xplane) can pass without indirect (or at least appreciably-indirect)reflections directly towards a concavely bent mirror surface, whilediverging beams of light in a second plane (for example the y-z plane)becomes a subject to indirect reflections between parallelmirror-structured surfaces, such as in an optical conductor.

Another technical problem is in being able to create, for a gas sensorthat has successfully solved one or more of the above technicalproblems, conditions whereby the light detector can receive from a lensor from a concavely bent mirror directly-reflected converging beams oflight with a high intensity, thereby enabling an improved analysis ofgases with a weak absorption pattern and/or low concentration.

Still another technical problem is in being able to create, by simplemeans, conditions whereby the light detector can assess directly- andindirectly-reflected beams of light, in order to allocateindirectly-reflected beams of light a longer measuring path thandirectly-reflected beams of light.

Given a gas cell and/or a gas sensor of the kind describe above, anothertechnical problem is in being able to realise the significance ofadjusting and arranging, in an x-z plane, opposing light-reflectingsurface sections in a cavity formed in a block, thereby enabling rays oflight to pass across said cavity, without reflections (or withoutappreciable reflections) in said x-z plane, after which they aredirectly reflected a predetermined number of times in opposingconcavely-bent mirror surfaces until a chosen measuring path is reachedand the directly-reflected rays of light can be aimed to be receivedwithin the cavity or to pass through said opening for exiting lightbeams or rays of light.

If a gas sensor and/or a gas cell of the kind described above is to beminiaturised, then another technical problem is in being able to realisethe significance of designing the cavity with two, ordinarily plane andparallel, light reflecting surfaces, close together, facing one another,and oriented parallel to the rays of light, creating conditions wherebyincoming rays of light in a y-z plane are indirectly reflected severaltimes before these diverging beams of light reach the concavely-bentmirror surfaces.

Another technical problem is in being able to realise the advantages andopportunities that are associated with designing at least a portion ofthe surface section in the x-y plane to present the shape of a grid.

Still another technical problem is in being able to realise thesignificance of allowing a gas sensor and/or a gas cell, in the shape ofa block, to be shaped, by simple means, by applying the principles usedfor producing compact disks.

Another technical problem is in being able to realise the significanceof, and the benefits that are associated with allowing said opposingconcavely-bent surface sections to be positioned according to theprinciples for a White mirror.

As relates to mass production, another technical problem is in beingable to create conditions whereby the opposing concavely-bent surfacesections and a plane surface are shaped in a block section, while theopposite plane surface is designed in the shape of a disk, and finally,that the block section and the disk may be fastened to one another usingknown techniques.

Yet another technical problem is in being able to realise thesignificance of, and the prerequisites that are associated with,allowing opposing surface sections and said two plane surfaces to becoated with gold, and of allowing the incoming light to be chosen fromwithin the frequency range for infrared light.

Another technical problem is in being able to provide a gas sensorand/or a gas cell that enables the use of a gas sample that can bepumped by means of a pump through the cavity, or that can be allowed todiffuse into the cavity.

Still another technical problem is in being able to show conditions forgas cells and the designed cavity, whereby the gas sample can be broughtinto or out of the cavity by means of recesses that lie beyond ameasuring path formed for the beams light and for these purposesrequired light-reflecting surfaces.

Another technical problem is in being able to provide a givenmeasurement from amongst one of several chosen measuring paths in thesame cavity with a predetermined size.

Finally, it should be considered a technical problem to realise theprerequisites for being able to apportion the measuring path into agiven number of segments, as well as to use the results from ameasurement as a reference determination.

Solution

In order to resolve one or more of the above technical problems, thepresent invention is based on an earlier design for a gas sensor and/orgas cell that has been adapted to enable a measurement of a gas samplethat is enclosed in a cavity, in the shape of a block; the wall or wallsections of the cavity exhibiting highly-reflective properties forlight; said cavity containing means, such as an opening for incomingrays of light, and means, such as an opening, through which directly-andindirectly-reflected light may be received within or exit the cavity.

Said rays of light are arranged to pass reflected across said cavity,before the reflected beams of light are aimed to be received within thecavity or to pass through said opening for exiting light.

Given a gas sensor of this kind, the present invention shows that saidcavity must exhibit opposing surface sections, arranged and coordinatedso close together that incoming rays of light are arranged to be able topass in a plane--without reflecting, or without appreciablyreflecting--across said cavity towards a concavely-bent mirror surfacethat is oriented in a right angle to the plane.

According to the preferred embodiments that lie within the scope ofinvention, the cavity is formed by two light-reflecting surfaces, placedclose together, facing one another in two parallel planes.

Moreover, the shape of said opposing surface sections is slightly bent.

Further, at least part of a surface section is processed to give it theshape of a grid.

Again, it is possible to allow incoming rays of light to be produced bya lamp with a narrow frequency range, and to allow the exiting rays oflight to be received by a light detector with an electrical and/or anelectronic circuit for analysing the present absorption spectrum.

By designing the gas sensor and/or the gas cell in the shape of a block,we are able to apply the principles that are used for producing compactdisks.

Moreover, within the scope of the invention, it is possible to positionopposing surface sections according to the principles for a Whitemirror.

In particular, the present invention shows that opposing surfacesections and a plane surface have the shape of a section of a block,whereas an opposite plane surface has the shape of a disk.

The invention shows that said opposing surface sections and said twoplane surfaces are coated with gold or another for lighthighly-reflective material. The frequency of the in-coming light ischosen to be in the range for infrared light.

According t o the invention, the gas sensor enables a gas sample to bepumped, by means of a pump, through the cavity, or the gas sensorpermits a gas sample to diffuse into the cavity.

Advantages

The primary advantage that characterises a gas sensor that exhibits thesignificant properties of the present invention is that conditions havebeen created whereby it is possible to miniaturise the externaldimensions of the gas sensor and/or the gas cell while at the same timecreating conditions for a long, adapted measuring path whose design,which may be mass-produced inexpensively, provides acceptably highprecision, in terms of measured results, by reducing the loss ofreflections in at least one plane, as well as by being able to create adivergence of reflected light beams towards the light detector.

The primary characteristic features of a gas sensor, according to thepresent invention, are set forth in the characterising clause of Claim1, below.

BRIEF DESCRIPTION OF THE DRAWINGS

Several exemplifying embodiments at present preferred for a gas cell anda gas sensor, having properties that are significant to the presentinvention, will now be described in greater detail with reference to theaccompanying drawings, where:

FIG. 1 shows a light emitter that sends diverging rays of light to aconcave mirror surface where the rays of light converge and arereflected towards a light receiver;

FIG. 2 shows an example of a previous design for a grid spectrometer;

FIG. 3 shows in a plane, an embodiment of a gas cell with a built-ingrid spectrometer;

FIG. 4 shows in a plane, another embodiment with a light emitter and alight receiver positioned next to one another;

FIG. 5 shows in a plane, an embodiment that applies the principles for aWhite mirror;

FIG. 6 shows the absorption characteristics of various gases.

DESCRIPTION OF PROPOSED EMBODIMENTS

FIG. 1 shows a gas sensor 1, which consists of a gas cell 2 with aconcavely-bent mirror 3, bent to the shape of a partial circle andpositioned in an x-y plane, with a mirror surface 3a.

The gas sensor 1 also comprises a light emitter 4 and a light receiver 5of known design.

The light receiver 5 is connected in a known way to equipment (notpictured) in order to analyse the absorption spectrum in the receivedbeams of light.

The vertical dimension (x-y plane) of the gas cell 2 is very small,being adapted to a selected light emitter 4 and light receiver 5. Withcurrent technology, the vertical height may realistically be between

0.1 and 0.5 mm.

With reference to FIG. 1, we see that the light emitter 4 is designed tosend diverging rays of light. We assume that the angle of divergence "a"is 40°.

The true angle of divergence may vary depending on the selected lightemitter and can hence be varied.

Nevertheless, the principle of the invention may be applied bycollimated light or by converging rays of light, by modifying the shapeof the mirror surface 3a and/or by changing the position of the lightreceiver.

While these embodiments will not be described in greater detail here, toa person skilled in the art, they represent common embodiments withassociated modifications.

The invention is based on the principle that the portion of light raysthat are oriented in the x-z plane are to pass, without reflection inthe indicated side surfaces 2c and 2d and/or the plane surfaces 2a and2b oriented in the x-z plane, to the convex mirror surface 3a, wherethey are directly-reflected, converging at the light detector 5.

The portion of the diverging rays of light that are oriented in the x-yplane will be able to pass--via recurring indirect reflections, in upperand lower plane surfaces 2a, 2b, to plane mirror surfaces positioned inthe x-y plane--to the convex mirror surface 3a positioned in the x-yplane.

Thus, FIG. 1 indicates that diverging, cavity-delimiting surfaces 2c, 2dare also light-reflecting, contributing with indirect reflection ofdiverging beams of light, which diverge at an angle that slightlyexceeds the angle for the surfaces 2c, 2d in the x-z plane.

The invention is based on producing beams of light from the lightemitter 4, which, while diverging in the x-z plane do not reflect in thex-z plane, but are instead directly-reflected in the mirror surface 3awith no practical loss by reflection, converging at the light receiver 5where they can be received and amplified.

The cavity 2 is delimited by the mirror surface 3a, two opposing planemirror surfaces 2a, 2b (a lower mirror surface 2a is shown; an uppermirror surface 2b has been removed to add clarity to the drawing), andthe diverging mirror surfaces 2c and 2d.

The radius of curvature for the mirror surface 3a of mirror 3 is chosenwith the centre 3C in the middle, between the light emitter 4 and thelight receiver 5.

A person skilled in this technique will understand that a betterexchange of light is obtained when the mirror surface 3a has a slightelliptical shape with its two focal points positioned in the lightemitter 4 and the light receiver 5.

It should be possible to increase the intensity of the light in thecircuit of the light receiver 5, provided the mirror surface 3a islonger in its y-axis than the input window of the circuit 5; the mirrorsurface 3a may even be somewhat concave in the x-y plane.

Ordinarily, the cone angle "a" of the light beams is greater than 20°but less than 60°; for example, between 30° and 50°, ordinarily 40°.

FIG. 2 illustrates that a beam "A" can be aimed at a mirror or a mirrorsection 6 with a grid surface 6a, thereby obtaining a grid spectrometer,inasmuch as the reflected beams A' and A" from an incoming beam A havedifferent wave lengths.

FIGS. 3-5 show in a plane, different designs for a gas cell or a gassensor, which exhibit the properties associated with the presentinvention.

Given this chosen projection, only directly-reflected light beams andrays of light are treated. The thickness of all embodiments is limitedin the y-axis.

According to FIG. 3, the embodiment shows a gas sensor 10 with a gascell whose shape is a cavity 2, having a delimited vertical dimension(y-axis), and a very broad horizontal dimension (x-z plane).

A mirror surface 13a and mirror surface 14a are angle related to oneanother.

The mirror surface 13a of the mirror 13 has the shape of a circular arcwith a radius from point 13C.

The mirror surface 14a of the mirror 14 has the shape of a circular arc.The radial point, which is not shown, with a radius slightly exceedingthat of mirror surface 13a.

When diverging rays of light are aimed into the cavity 2 through aninlet 15 by means of a source (not pictured) of diverging light rays inthe opening 15, the entire mirror surface 13a is illuminated, and thelight beams are reflected and collimated towards a grid surface 16a fromwhich they are diffracted to a focusing surface 14a that reflects thefocused wave length sections through openings 20a, 20b, . . . , 20f,each adapted to a selected wave length range, regardless of whether ornot the wave length has been absorbed.

If the cavity contains the light receivers then these are positioned atthe same places as the openings 20a, 20b, . . . 20f.

FIG. 4 is meant to illustrate an alternative embodiment in which a lightemitter 4 aims diverging rays of light at a mirror 25 whose surface 25ais bent like a circular line with a centre point 25C.

The reflected beams of light strike a plane mirror surface 26a and arereflected across to a plane mirror surface 27a, which in turn reflectsthe beams of light to the mirror surface 25a that converges the beams oflight at the light receiver 5.

Obviously, the various embodiments shown in FIGS. 1, 3 and 4 givedifferent lengths of direct and indirect measuring paths for the lightwithin the cavity 2.

The gas that is to be analysed is introduced through holes or openingsoutside the area for the flood of light. Exactly where these arepositioned must be answered on a case-by-case basis. In the embodimentsshown, outlets or holes are indicated by the references 30 and 31.

FIG. 5 shows an embodiment that exploits the principles for Whitemirrors.

The gas sensor 1 has been adapted to enable a measurement of a gassample that is enclosed in a cavity 2.

The gas sensor 1 has the shape of a block 50. Each wall or wall sectionof the cavity 2 exhibits highly reflective properties for light.

Said cavity 2 contains means 51, in this case an opening for incomingdiverging light, and means 52, in this case an opening 52 for exitingconverging light.

Significant for the present invention is that said cavity 2 in block 50must contain opposing mirror-related surface sections, such as surfacesections 55a, 56a, and 57a, where the surface sections are designed andcoordinated so that incoming rays of light 60 are arranged to pass apredetermined number of times--without indirect reflection in the x-zplane, but with indirect reflection in the x-y plane--across said cavity2 in the x-z plane, thereby providing, in spite of the miniaturisation,a given measuring path before the reflected converging light 61 is aimedto pass through said opening 52 for exiting light.

Also significant for the present invention is that the cavity 2 isformed by two plane light-reflecting surfaces, placed close together,facing one another, parallel in the x-z plane. One surface, in the block50 is designated 2a. The other surface 2b has the shape of a disk (notpictured).

Said surfaces 2a, 2b are positioned very close to one another, with thedistance between them adjusted to the size of the cross-section for thethermal light area for the light emitter 4. In practice, this distanceshould equal the surface of the light source, which is equal to, orgreater than, the surface of the detector.

Further, according to the embodiment of the invention shown in FIG. 5,said opposing surface sections 55a, 56a and 57a are given a slightlybent circular shape.

Part, if not all, of a surface section 57a is processed to exhibit agrid shape, thereby creating an internal grid spectrometer in the block50.

However, there is no reason why the receiver 5 may not make up, orcomprise, a grid spectrometer of this kind.

The invention also shows that the incoming rays of light 60 are producedby a sending organ 4, which has the shape of a lamp with a narrowfrequency range.

The exiting rays of light 61 are received by a light detector 5 with acircuit 10 for analysing the absorption spectrum, which is treated in aunit 11, and presented using known methods, via a cable 12, on adisplay.

The present invention strongly proposes that the block 50 be designedaccording to the principles used for producing compact disks, with athickness in the order of 0.3 mm.

According to FIG. 5, the embodiment shows that said opposing surfacesections 55a, 56a and 57a are positioned according to the principles fora White mirror.

FIG. 5 illustrates that the opposing surface sections 55a, 56a, 57a anda plane surface 2a are shaped out of a block section 50, whereas theopposing plane surface 2b is shaped as a disk.

Said opposing surface sections and said two plane surfaces are coatedwith gold. The incoming rays of light 60 are chosen to be within thefrequency range for infrared light.

Said gas sample is introduced into the cavity by means of holes 30 and31. The gas can be introduced via a pump (not pictured), or by diffusionin the cavity.

According to FIG. 5, the embodiment also shows that the mirror surface55a of the mirror 55 has its circular centre 55C in the mirror surface56a of the mirror 56, and that the diverging rays of light 60 aredirectly reflected, converging from the mirror surface 55a, to appear asa point 60' on the mirror surface 56a.

The point 60' is projected, divergently, to the mirror surface 57a ofthe mirror 57, and is directly reflected, convergently, back to themirror surface 56a as a point 61, situated the same distance from thepoint of focus 57C as point 60', but on the other side of the point offocus 57C.

Point 61 is then directly reflected to the mirror surface 55a andreflected again, convergently, as a point 62 on the mirror surface 56a.

Thereafter, point 62 is directly reflected, divergently, in the mirrorsurface 57a, which directly reflects the beams of light 561 causing themto converge at the light detector 5.

From this description we see that the mirror surface 56a could easily bebroken up to have only point-related (60', 61, 62) reflectiveproperties. The areas in between these points could server as gas inletsand outlets (30, 31).

FIG. 6 shows the known absorption tendencies of various gases atdifferent wave lengths, as well as the absorption tendencies of certaingases within specific ranges of wave lengths.

It is possible, within the scope of the invention, to produce a gas cellthat is inexpensive, preferably from a plastic material that containsrecesses for enclosing the light emitters and the light receiver.

If a ceramic material were used to produce the gas cell, then this samematerial could also be used to form the required electronic circuit forthe gas sensor.

We should note in particular that since the light receiver 5 will beable to receive beams of light with different measuring paths, acentrally positioned image 560' from the directly reflected beams oflight, and adjacent images 561, 561', as well as 562, 562', and soforth, with increasing measuring paths that correspond to the distancefrom the centrally positioned image 560', it will be possible, using thesame design for the cavity 2, to choose any given measuring path. Themeasuring path is increased by an increasing diverging angle for theincoming light; for example, 45° to 60°.

If we analyse the received light spectrum for several gases, then as areference, it is significant that we analyse the intensity of light forwave lengths without absorption as well, thereby providing compensationfor changes in the lamp used.

The analysed light intensity can be written according to the followingformula:

    I=Io.e.sup.-s.c.1

where

Io=intensity before absorption;

s=the absorption cross-section;

c=the concentration;

l=the length of the optical measuring path.

By means of the proposed embodiments, in particular the embodiment shownin FIG. 5, it is possible to analyse the intensity of light for a chosenwave length, given a first predetermined optical length, such as inpoint 560', and a second predetermined optical length, such as in point562, whereby, through calculation, the value "c" can be determinedwithout consideration for the value Io.

Thus, the same wave length can be used for different measuring paths,thereby establishing a reference while using the same optical filterwith the same optical properties.

Obviously, the invention is not restricted to the exemplifyingembodiments described above. Instead, modifications can be made withinthe scope of the inventive thought defined in the following Claims.

I claim:
 1. A gas sensor, designed to enable a measurement of a gassample that is enclosed in a cavity (2), having the shape of a block;the wall or wall sections of the cavity exhibiting highly-reflectiveproperties for light; said cavity containing means (51) for incomingrays of light (60), where said rays of light are arranged to passreflected across said cavity (2), thereby forming an optical measuringpath before the reflected beams of light are aimed towards means (52)for exiting rays of light, characterised in that said cavity (2)exhibits opposing surface sections (2a, 2b) that are designed andcoordinated so close together that incoming rays of light (60) arearranged to pass in a plane--without reflecting, or without appreciablyreflecting--across said cavity (2) to a concave mirror surface (55a,56a, 57a) that is oriented in a right angle to the plane and thatreflected beams of light are converging at said means (52) for exitingrays of light.
 2. According to claim 1, a gas sensor characterised inthat the cavity (2) is formed with two light-reflective surfaces (2a,2b), placed close together, facing one another in parallel planes. 3.According to claim 1, a gas sensor characterised in that the shape ofsaid opposing surface sections (2a, 2b) is slightly bent.
 4. Accordingto claim 3, a gas sensor characterised in that at least part of asurface section has been processed to present the shape of a grid. 5.According to claim 1, a gas sensor characterised in that the incomingrays of light (60) are produced by a light emitter with a narrowfrequency range.
 6. According to claim 1, a gas sensor characterised inthat the exiting rays of light are received by a light detector with acircuit for analysing the current absorption spectrum.
 7. According toclaim 1, a gas sensor characterised in that the block is formedaccording to the principles that are used for making compact disks. 8.According to claim 1, a gas sensor characterised in that said opposingsurface sections (55, 56 and 57) are oriented according to theprinciples for a White mirror.
 9. According to claim 1, a gas sensorcharacterised in that the opposing surface sections and one planesurface (2a) are formed in of a block, whereas the opposite planesurface (2b) is formed into a disk.
 10. According to claim 1, a gassensor characterised in that said opposing surface section and said twoplane surfaces are coated with gold, and that the frequency of theincoming light (60) is chosen to be in the range for infrared light. 11.According to claim 1, a gas sensor characterised in that said gas sampleis arranged to be pumped through the cavity (2) by means of a pump. 12.According to claim 1, a gas sensor characterised in that said gas sampleis arranged to diffuse into the cavity (2).
 13. According to claim 1, agas sensor characterised in that the reflected rays of light within thecavity (2) are converging and diverging.
 14. According to claim 1, a gassensor characterised in that the cavity is formed in a plastic orsilicon disk.
 15. According to claim 1, a gas sensor characterised inthat a light receiver (5) is designed to receive beams of light of agiven wave length with one of several accessible lengths of measurement.16. According to claim 1, a gas sensor characterised in that one and thesame wave length can be assessed for different lengths of measurement,thereby establishing a reference.
 17. According to claim 2, a gas sensorcharacterised in that the shape of said opposing surface sections (2a,2b) is slightly bent.
 18. According to claim 17, a gas sensorcharacterised in that at least part of a surface section has beenprocessed to present the shape of a grid.